Apache Module mod_cache

Summary

This module is experimental. Documentation is still under development...

mod_cache implements an RFC 2616 compliant HTTP
content cache that can be used to cache either local or proxied content.
mod_cache requires the services of one or more storage
management modules. Two storage management modules are included in
the base Apache distribution:

implements a memory based storage manager.
mod_mem_cache can be configured to operate in two
modes: caching open file descriptors or caching objects in heap storage.
mod_mem_cache can be used to cache locally generated content
or to cache backend server content for mod_proxy when
configured using ProxyPass
(aka reverse proxy)

Content is stored in and retrieved from the cache using URI based keys. Content with
access protection is not cached.

The CacheDefaultExpire directive specifies a default time,
in seconds, to cache a document if neither an expiry date nor last-modified date are provided
with the document. The value specified with the CacheMaxExpire
directive does not override this setting.

The CacheEnable directive instructs
mod_cache to cache urls at or below
url-string. The cache storage manager is specified with the
cache_type argument. cache_type mem
instructs mod_cache to use the memory based storage
manager implemented by mod_mem_cache.
cache_typedisk instructs
mod_cache to use the disk based storage manager
implemented by mod_disk_cache.
cache_typefd instructs
mod_cache to use the file descriptor cache implemented
by mod_mem_cache.

In the event that the URL space overlaps between different
CacheEnable directives (as in the example below),
each possible storage manager will be run until the first one that
actually processes the request. The order in which the storage managers are
run is determined by the order of the CacheEnable
directives in the configuration file.

Ordinarily, if a request is cancelled while the response is being
cached and delivered to the client the processing of the response will
stop and the cache entry will be removed. The
CacheForceCompletion directive specifies a
threshold beyond which the document will continue to be cached to
completion, even if the request is cancelled.

The threshold is a percentage specified as a value between
1 and 100. A value of 0
specifies that the default be used. A value of 100
will only cache documents that are served in their entirety. A value
between 60 and 90 is recommended.

Ordinarily, documents with no-cache or no-store header values will not be stored in the cache.
The CacheIgnoreCacheControl directive allows this behavior to be overridden.
CacheIgnoreCacheControl On tells the server to attempt to cache the document
even if it contains no-cache or no-store header values. Documents requiring authorization will
never be cached.

According to RFC 2616, hop-by-hop HTTP headers are not stored in
the cache. The following HTTP headers are hop-by-hop headers and thus
do not get stored in the cache in any case regardless of the
setting of CacheIgnoreHeaders:

Connection

Keep-Alive

Proxy-Authenticate

Proxy-Authorization

TE

Trailers

Transfer-Encoding

Upgrade

CacheIgnoreHeaders specifies additional HTTP
headers that should not to be stored in the cache. For example, it makes
sense in some cases to prevent cookies from being stored in the cache.

CacheIgnoreHeaders takes a space separated list
of HTTP headers that should not be stored in the cache. If only hop-by-hop
headers not should be stored in the cache (the RFC 2616 compliant
behaviour), CacheIgnoreHeaders can be set to
None.

Example 1

CacheIgnoreHeaders Set-Cookie

Example 2

CacheIgnoreHeaders None

Warning:

If headers like Expires which are needed for proper cache
management are not stored due to a
CacheIgnoreHeaders setting, the behaviour of
mod_cache is undefined.

Ordinarily, documents without a last-modified date are not cached.
Under some circumstances the last-modified date is removed (during
mod_include processing for example) or not provided
at all. The CacheIgnoreNoLastMod directive
provides a way to specify that documents without last-modified dates
should be considered for caching, even without a last-modified date.
If neither a last-modified date nor an expiry date are provided with
the document then the value specified by the
CacheDefaultExpire directive will be used to
generate an expiration date.

In the event that a document does not provide an expiry date but does
provide a last-modified date, an expiry date can be calculated based on
the time since the document was last modified. The
CacheLastModifiedFactor directive specifies a
factor to be used in the generation of this expiry date
according to the following formula:
expiry-period = time-since-last-modified-date * factor
expiry-date = current-date + expiry-period
For example, if the document was last modified 10 hours ago, and
factor is 0.1 then the expiry-period will be set to
10*0.1 = 1 hour. If the current time was 3:00pm then the computed
expiry-date would be 3:00pm + 1hour = 4:00pm.
If the expiry-period would be longer than that set by
CacheMaxExpire, then the latter takes
precedence.

The CacheMaxExpire directive specifies the maximum number of
seconds for which cachable HTTP documents will be retained without checking the origin
server. Thus, documents will be out of date at most this number of seconds. This maximum
value is enforced even if an expiry date was supplied with the document.